1. Field of the Invention
The present invention generally relates to thermally decomposable organometallic compounds and complexes which are useful in chemical vapor deposition (CVD) processes, for formation of metal films on substrates.
2. Description of the Related Art
Chemical vapor deposition is widely used for the formation of metal films on a variety of substrates. CVD is a particularly attractive method for forming metal films because it is readily scaled up to production runs and because the electronics industry has a wide experience and an established equipment base in the use of CVD technology which can be applied to CVD processes.
CVD requires source reagents which are sufficiently volatile to permit their gas phase transport into the decomposition reactor. The source reagent must decompose in the CVD reactor to deposit only the desired element(s) at the desired growth temperature on the substrate. Premature gas phase reactions are desirably avoided, and it generally is desired to controllably deliver source reagents into the CVD reactor to effect correspondingly close control of stoichiometry.
Many potentially useful metals do not form compounds which are well suited for CVD. Although some source reagents are solids which are amenable to sublimation for gas-phase transport into the CVD reactor, the sublimation temperature may be very close to decomposition temperature. Accordingly, the reagent may begin to decompose in the lines leading to the CVD reactor, and it then becomes difficult to control the stoichiometry of the deposited films.
Accordingly, there is a continuing search in the art for improved source reagent compositions which are amenable to vaporization to form the source component vapor for CVD processes, for applications such as the formation of diffusion barriers, conductors, dielectrics, protective coatings, phosphors, electroluminescent structures, ferroelectrics, giant magnetoresistive films, corrosion-resistant films, and mixed metal films.
In the chemical vapor deposition of multicomponent material systems, multiple source reagents are delivered to the CVD reactor. A particularly advantageous way of delivering multiple source reagents is to accurately mix neat liquid source reagents or liquid solutions of source reagents and then flash vaporize the mixture and deliver the resulting vapor to the reactor. It is possible in this situation for the reagents to undergo reactions, either in the liquid phase before vaporization or in the gas phase after vaporization. If these reactions convert a source reagent to an insoluble or non-volatile product, or to a material of different chemical or physical properties, then the elements contained in that product will not reach the substrate and the stoichiometry of the deposited film will be incorrect.
Examples of this problem (wherein Et is ethyl; tBu is tert-butyl; iPr is isopropyl; and thd is tetramethylheptanedionate) include the following:
(i) during deposition of PbZr.sub.x Ti.sub.1-x O.sub.3, using (Et).sub.4 Pb, Zr(OtBu).sub.4, and Ti(OiPr).sub.4 source reagents, ligand exchange between the Zr and Ti reagents resulted in formation of Zr(OiPr).sub.4 (and perhaps other products of which Zr(OiPr).sub.4 is a monomer), which had very low volatility and which condensed in the gas manifold or vaporizer; PA1 (ii) when solutions of Ba(thd).sub.2 and Ti(OiPr).sub.4 were mixed prior to vaporization, an insoluble precipitate was formed, presumably Ba(OiPr).sub.2 ; and PA1 (iii) when solutions of Pb(thd).sub.2 and Ti(OiPr).sub.4 were mixed in butyl acetate, the reagents reacted to form compounds of differing physical properties, such as Pb(OiPr).sub.2 and Ti(OiPr).sub.2 (thd).sub.2. PA1 Sr(thd).sub.2 ; Ta(OEt).sub.5 ; and Bi(Ph).sub.3 wherein Ph=phenyl, PA1 (i) 2,2,6,6-tetramethyl-3,5-heptanedionate; PA1 (ii) 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate; PA1 (iii) 1,1,1,2,2,3,3-heptafluoro-7,7-dimethyl-4,6-octanedionate; PA1 (iv) cyclopentadienyl; PA1 (v) 4,4'-(ethane-1,2-diyldiimino) bis (3-pentene-2-one); PA1 (vi) pentamethylcyclopentadienyl and other substituted cyclopentadienyls; PA1 (vii) 2,4-pentanedionate; and PA1 (viii) 1,1,1-trifluoro-2,4-pentanedionate. PA1 (i) oxyhydrocarbyl ligands; PA1 (ii) nitrogenous oxyhydrocarbyl ligands; PA1 (iii) fluorooxyhydrocarbyl ligands; and PA1 (iv) thiooxyhydrocarbyl ligands. PA1 (a) amines and polyamines; PA1 (b) bipyridines; PA1 (c) ligands of the formula: ##STR1## wherein G is --O--, --S--, or --NR--, wherein R is H or hydrocarbyl; (d) crown ethers; PA1 (e) thioethers; and PA1 (f) ligands of the formula: EQU R.sup.0 O(C(R.sup.1).sub.2 C(R.sup.2).sub.2 O).sub.n R.sup.0 PA1 R.sup.0 =H, methyl, ethyl, n-propyl, cyanato, perfluoroethyl, perfluoro-n-propyl, or vinyl; PA1 R.sup.1 =H, F, or a sterically acceptable hydrocarbyl substituent; PA1 R.sup.2 =H, F, or a sterically acceptable hydrocarbyl substituent; PA1 n=2, 3, 4, 5, or 6; and PA1 R.sup.1 =H, methyl, ethyl, n-propyl, cyanato, perfluoroethyl, perfluoro-n-propyl, or vinyl; PA1 R.sup.1 =H, F, or a sterically acceptable hydrocarbyl substituent; PA1 R.sup.2 =H, F, or a sterically acceptable hydrocarbyl substituent; PA1 n=2, 3, 4, 5, or 6; and PA1 (i) a metalorganic complex of the formula: EQU MA.sub.Y X PA1 (ii) a solvent or suspending agent therefor. PA1 (i) at least one metal coordination complex including a metal to which is coordinatively bound at least one ligand in a stable complex, wherein the ligand is selected from the group consisting of: .beta.-diketonates, .beta.-thioketonates, .beta.-ketoiminates, .beta.-diiminates, C.sub.1 -C.sub.8 alkyl, C.sub.2 -C.sub.10 alkenyl, C.sub.2 -C.sub.15 cycloalkenyl, C.sub.6 -C.sub.10 aryl, C.sub.1 -C.sub.8 alkoxy, and fluorinated derivatives thereof; and PA1 (ii) a solvent for the metal coordination complex. PA1 (i) at least one metal coordination complex, each of such metal coordination complexes including a metal coordinatively binding at least one ligand in a stable complex, wherein such at least one ligand is selected from the group consisting of .beta.-diketonates and .beta.-ketoesters, and their sulfur and nitrogen analogs (i.e., corresponding ligands containing S or N atoms in place of the O atom(s) in the .beta.-diketonates and .beta.-ketoesters); and PA1 (ii) a solvent for such metal coordination complex(es). PA1 a, x, M and R are as defined hereinabove; PA1 (i) at least one, and preferably at least two, metal coordination complexes, each of the formula: EQU M.sup.i A.sub.a (OR).sub.x B.sub.y Q.sub.z PA1 (ii) a solvent for the metal coordination complex(es). PA1 (i) at least one, and preferably at least two, metal coordination complexes, each of the formula: EQU M.sup.i A.sub.a (OR).sub.x B.sub.y Q.sub.z PA1 (ii) a solvent for the metal coordination complex(es).
Another specific example illustrating this problem is the preparation of films of strontium bismuth tantalate and strontium bismuth niobate (SrBi.sub.2 Ta.sub.2 O.sub.9 and SrBi.sub.2 Nb.sub.2 O.sub.9) by CVD for use in non-volatile ferroelectric random access memories. The most commonly used strontium source reagents are .beta.-diketonate complexes such as Sr(thd).sub.2. When a solution is heated containing the following source reagents for deposition of SrBi.sub.2 Ta.sub.2 O.sub.9 :
the ethoxide ligands of the tantalum reagent exchange with the thd ligands of the strontium reagent, leading to the formation of undesirable strontium alkoxide species that have reduced volatility and that can decompose in the vaporization zone. Alternatively, when these reagents are provided separately in bubblers, similar ligand exchange reactions occur in the gas phase; the resulting solids constrict the gas lines or alter the film stoichiometry.
In certain instances, such problems can be avoided by using identical ligands on the metals to make ligand exchange a degenerate reaction (i.e., where the exchanging ligand is identical to the original ligand). Examples of this approach include the use of tetraethylorthosilicate, triethylborate and triethylphosphite for deposition of borophosphosilicate glasses (J. Electrochem. Soc., 1987, 134(2), 430). In many instances, however, this method for avoiding the problem is not possible because the appropriate compound does not exist, is too unstable or involatile to be used for CVD, or otherwise has disadvantageous physicochemical material properties. For example, for deposition of PbZrxTi.sub.1-x O.sub.3, a reagent system with identical ligands is problematic because while Pb(thd).sub.2 and Zr(thd).sub.4 are stable and volatile, Ti(thd).sub.4 does not exist and Ti(thd).sub.3 is extremely air sensitive. Similarly, while Ti(OtBu).sub.4 and Zr(OtBu).sub.4 are stable and volatile, Pb(OtBu).sub.2 is thermally unstable at temperatures required for volatilization.
The foregoing problems are also encountered in the circumstance where the metal source reagent is provided in a liquid solution and the solvent contains moieties which react with ligands of the source reagent compound to produce undesirable ligand exchange reaction by-products which display different physical properties and are involatile or insoluble.
Accordingly, it is an object of the present invention to provide improved metal source reagent compositions for the deposition of corresponding metals and metal oxides via CVD processes.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.